Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in the United States, Australia, or elsewhere on or before the priority date of the disclosure herein.
Electricity generation, transmission, distribution and consumption, is ideally by way of three phase systems. These systems generally comprise high and very high voltages in order to transmit the electrical energy in a cost effective manner. End consumers are generally connected to a low voltage distribution supply voltage of less than 1000V, which is achieved by way of step-down transformers installed at various locations along the supply chain.
In areas having relatively small consuming loads, connection to the distributor may be by way of a single phase system having, for example, two wires. Alternatively, there may be two or three phase power available via three or four wire connections to end users.
In relatively large geographical areas, with known relatively small loads, and no, or low, demand growth anticipation, the high/medium voltage distribution may be supplied by medium or high voltage single phase systems, known as: single phase (two “actives” only); or, SWER; or, by Dual SWER systems.
SWER systems are favoured over other supply systems in many jurisdictions including Australia, New Zealand, Canada, Brazil and South Africa, as they are more economical and only require as little as a single aerial wire—the electrical loop being completed by a convenient ‘earth return’ system utilising earth rods embedded into the ground at various locations along the supply chain.
There are of course many circumstances where it is required, or at least desired, to operate a three phase load, such as a three phase motor, in remote or rural areas where only, for example, conventional 120-volt or 240-volt single phase power is available. For example, rural properties such as dairy farms often require loads bigger than 3 kW, for operating pumps, refrigeration units, air conditioning systems, etc. However, if such properties are only supplied with power via, for example, a (single) SWER system, three phase power is not available unless suitable three phase conversion systems can be readily provided.
Furthermore, as electricity supply is regulated in most countries, the electricity industry's main concern is to provide continuous reliable supply. In addition to the reliability of supply, another major concern is for the supply to be within the specified parameters of power quality standards, i.e. stable voltage levels within certain limits; VTHD (Voltage Total Harmonic Distortion) within strict limits; and/or, other quality parameters. These standards generally apply to all supply systems, whether single or multiple phase. Hence, whether end users of three phase power draw balanced or imbalanced power from a distribution network is of utmost importance. As a result of this, three phase consumers are often connected via four wire three phase systems for network stability.
All of the abovementioned electrical supply systems are in the form of alternating current (“AC”) systems. For AC systems, as current alternates in time, the directions of the phases keep on changing, hence this change of vector direction in time is commonly termed “Phasor”. For three phase systems, the respective phasors can be described as follows:Va=A*sin(ωt+0°);  (a)Vb=B*sin(ωt+120°);  (b)Vc=C*sin(ωt+240°);  (c)Where; ωt+0° describes an angle that keeps changing in time with an offset of 0° from a known reference; ωt+120° describes an angle that keeps changing in time with an offset of 120° from the same known reference; and, ωt+240° describes an angle that keeps changing in time with an offset of 240° from the same known reference. The frequency of alternations is determined by ω (radians/sec); and A, B, & C represent the amplitude of the voltage value. In this example A=B=C=Unity.
The shape of these phasors (i.e. Va,Vb,Vc) and the angle references (i.e. (a),(b),(c)) can be seen in FIGS. 3 & 4, which will be described in further detail later in this description.
It should be apparent from the above discussion that if a three phase power supply is to be produced from a single phase source then two phases must be reconstructed. For example, if Va is given, Vb and Vc will need to be reconstructed.
Conversion systems exist for operating three phase induction motors in single phase environments. For example, U.S. Pat. No. 4,618,809, to Naoyuki Maeda, describes an inverter apparatus for converting a single phase AC power supply into a three phase AC power supply for operating a three phase motor. Although successful at producing a three phase supply for operating an induction motor, this solution has many limitations, the most notable of which is the use of a household AC supply source which is not referenced to a common neutral, and hence, which does not readily allow the generated additional phases to be utilised as separate single phases.
Further examples of similar convertor apparatus include: U.S. Pat. No. 4,644,241, also to Naoyuki Maeda; U.S. Pat. No. 4,908,744, to Thomas G. Hollinger; U.S. Pat. No. 5,969,957, to Divan, et al.; AU Patent No. 2006329365, to Abdallah Mechi; U.S. Pat. No. 5,272,616, to Divan, et al.; U.S. Pat. No. 6,297,971, to Larry G. Meiners; and, U.S. Pat. No. 6,831,849, to Fowler, et al.
For many reasons the solutions proposed in these citations are also considered to be insufficient for producing reliable and/or useful three phase power supplies. Like Maeda's first U.S. Pat. No. 4,618,809, none of these additional solutions disclose or suggest the use of a common neutral. Many electricity distribution companies utilise a common neutral as the fourth wire in a three phase distribution system for network stability, as was described above. Hence, none of these solutions could readily be integrated into such four wire three phase systems.
Aside from the above conversion systems for operating three phase induction motors in single phase environments, other systems for simply converting a single phase supply into a three phase supply are available. Such solutions typically use one of two methods for conversion, these being, the coil-capacitor method, or the three phase electric motor method.
As the name suggests, the coil-capacitor method reconstructs the second and third phases using coils and capacitors. The additional coils raise the voltage, whilst the capacitors are used to shift the phase. This method is limited to specific loads, and has to be adjusted when load impedance is changed. Hence, this method is not ideal for producing a stable three phase supply.
The three phase electric motor method uses the motor to construct the required three phases as the rotating motor acts like a generator. However, as the power capacity is determined by the size of the electric motor, this solution again has limited usage in electricity distribution supply chains.
A need therefore exists for alternative and/or improved methods and apparatus for converting a single phase power supply source into a three phase power supply.